Tracking nitrates

Historically, the main industry on Barbados has been agriculture, notably sugar cane, but in recent years this has been overtaken as the principal money-earner by tourism. Tourism is largely centred on the south-west coast,

and along the west coast to the north of Bridgetown. In contrast, the east coast is less developed and exposed to rolling Atlantic waves.

Q1C1 + Q2C2 + Q3C3 = Q4C4 where Q = flowrate and C = nitrate concentration

A 1km wide test section was selected on the moderately developed south-east coast. The question posed was – can the observed rise in nitrates from inland to the coastline be attributed to wastewater disposal? The unknown in the equation is therefore C3 – the nitrate concentration in wastewater.

Once calculated, this figure can be compared to the expected concentration based on available data. Estimated or known parameters are:

• groundwater inflow and recharge to the coastal zone;
• wastewater discharge, based on population density in the coastal zone combined with a demand of 150l/capita/day;
• nitrate concentrations in groundwater as measured at inland and coastal sampling points.

The value for C3 calculated from the equation was 77mg/l. In Barbados, the nitrogen loading per capita is 5kg/year. Assuming that this is all oxidised to nitrate in the groundwater environment, then such a loading diluted by the per capita wastewater discharge gives a nitrate concentration of 90mg/l in the effluent. Thus, the two values are of the same order.

The methodology was then applied to a more populous zone at South Point. This time, the value of 77mg/l was assigned for nitrate in wastewater and the equation was solved for the resultant nitrate concentration in groundwater at the coastline before discharge. The calculated concentration was 11mg/l. This compares with the average observed value of 10.7mg/l at a coastal monitoring borehole.

Conclusions

• the typical concentration of nitrate in groundwater beneath rural areas is 7-8mg/l, most of which (c.90%) is due to fertiliser application;
• moderate coastal development (<2000 population per km2) increases nitrates in groundwater discharging at the coastline by about 30%;
• only when population densities exceed 3000/km2, or where groundwater outflow is relatively low does the wastewater contribution to nitrate in groundwater match and exceed that from fertilisers; total nitrate concentrations can be up to 25mg/l.